Polyacrylates, made by homopolymerizing acrylic and substituted acrylic ester monomers, are versatile resins. Of particular importance among them is poly(methyl methacrylate) made by free-radical polymerization of methyl methacrylate monomer as shown below. ##STR1##
Poly(methyl methacrylate) is used to make the so-called "organic glass" tradenamed Plexiglas.RTM. plastic by the Rohm & Haas Company. It is a hard and fairly rigid material which can be sawed, carved, or worked on a lathe. When heated above its glass transition temperature, poly(methyl methacrylate) is a tough, pliable, and extensible material that is easily bent or formed into complex shapes that can be molded or extruded. It finds many applications in which its shatter-resistance, high optical clarity, and workability make it preferred over glass. Among its important applications are airplane windows, electronic displays, safety shields, signs, see-through cabinet doors, automotive taillights, complex camera lenses, magnifiers, and reducers.
Though poly(methyl methacrylate) is probably the most prominent member of the acrylate polymers, other acrylates also find a wide range of commercial applications such as in coatings, adhesives, textile and paper sizes, etc. The choice of the polymer composition depends upon the intended use. Thus acrylic esters of higher alcohols impart softness or rubbery character, and the alpha-methyl group in methyl methacrylate imparts stability, hardness, and stiffness to the resultant polymers Copolymers of acrylic esters with other ethylenically unsaturated monomers such as styrene, acrylonitrile, and vinyl acetate can also be made and they offer interesting properties and applications.
Versatility not withstanding, the polyacrylate resins suffer from one major disadvantage--they are very much prone to develop static when rubbed against other materials including air. Though static accumulation is by no means an unique property of polyacrylates, because all plastics by virtue of their insulative properties by and large are susceptible to static buildup, the polyacrylates are among those that are most susceptible. Thus the surface resistivity of poly(methyl methacrylate) is 1.times.10.sup.17 to 2.times.10.sup.18 ohms/square which is higher than that of other plastics which typically lie in the 5.times.10.sup.13 to 1.times.10.sup.15 ohms/square range.
Static presents many problems to plastics Among them are dust attraction, electrical shock, fire hazard through spark generation, etc. Dust attraction by static in plastic is a major nuisance, but more than that, when one tries to wipe off this tenaciously adhering powder by buffing, it acts as an abrasive and scratches the surface of the plastic. Thus, one may start with a perfectly transparent Plexiglas.RTM. plastic object but with use, soon end up with one that is hazy and opaque.
The recognition of the nuisance and other problems associated with static in plastic is not new and various approaches for controlling static buildup have been described in the literature. The most important of these approaches consists of treating the outer surface of the plastic with an antistatic compound, called an antistat in short. The Modern Plastics Encyclopedia lists 170 commercial antistatic additives. Another approach involves adding conductive pigments such as carbon black or metal fibers to the bulk of the plastic.
The second approach is often undesirable and is particularly so with polyacrylates such as Plexiglas .RTM. plastic since it reduces transparency, aesthetic appeal, and deteriorates physical properties of the plastic. This approach is also undesirable because it increases bulk conductivity and as such minimizes one of the major attributes of plastic over metal, namely its insulative property.
The major problem with the use of topical antistats, on the other hand, is that the treatments are nondurable and the antistatic protection is lost in the course of normal use of the article or when the articles are washed.
In certain instances the antistats are directly incorporated into the bulk of the polymer to impart permanent antistatic properties. An example is the use of lauryl diethanolamide, the structure of which is shown below, ##STR2## as an internal antistat in polyethylene film used for electronic packaging.
Internal antistats are used in relatively large concentrations and they work by gradual migration to the surface to form a thin equilibrium layer on the surface of the article, due to their inherent incompatibility with the resin. Even though internal antistats are used to provide permanent protection against static, they do not always work reliably or as desired. For instance, when polyethylene films containing lauryl diethanolamide are washed with water, the antistatic protection is lost and does not reappear for a long time presumably due to the loss of the surface layer via washing and the relatively slow rate of migration that helps to form a new layer.
Still another problem with internal antistats is that they act as plasticizers and as such deteriorate the physical properties of the plastic. Thus, with plastics like poly(methyl methacrylate) with relatively low glass transition temperatures (110.degree.-115.degree. C.), further decrease in softening temperature through plasticization is particularly undesirable.
When optical clarity is of prime value, as is in the case of Plexiglas.RTM. poly(methyl methacrylate) plastic, the use of internal antistats is additionally undesirable because they reduce such clarity.